1. Field of the Invention
The present invention relates to a single cell unit design for a solid oxide fuel cell (SOFC), and more particularly, to a SOFC comprised of a plurality of pre-sintered single cell units which are connected in electrical series, whereby a single cell unit may be individually examined for defects and replaced without permanently impairing the performance of the SOFC stack.
SOFCs have been the subject of considerable study. Representative of this technology are U.S. Pat. Nos. 4,476,198 to Ackerman, et al.; 4,510,212 to Fraioli; 4,761,349 to McPheeters, et al.; 4,857,420 to Maricle et al.; and 5,330,859 to McPheeters, et al.; wherein each relate to certain aspects of SOFCs.
2. Background of the Invention
Fuel cells are electrochemical systems which generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surface of electrodes. Solid oxide fuel cells (SOFCs), which are fuel cells having electrolyte material in a solid form, are generally comprised of a stack of individual cells which are connected in electrical series to generate a useful voltage. In a conventional SOFC design, four materials are used to fabricate each single cell unit: anode, cathode, electrolyte, and interconnect material. Within each cell, the solid electrolyte material separates the anode and cathode materials, which comprise the electrodes. The interconnect material electronically connects the anode of one cell with the cathode of an adjacent cell.
Several configurations for SOFCs have been developed, including the tubular, flat plate, and monolithic designs. In a tubular design, each single cell unit includes electrode and electrolyte layers applied to the periphery of a porous support tube. While the inner cathode layer completely surrounds the support tube, the solid electrolyte and outer anode structures are discontinuous to provide a space for the electrical interconnection of the cell to the outer surface of parallel cells. Fuel gas is directed over the exterior of the tubular cells, and oxidant gas is directed through the interior of the tubular cells.
The flat plate design incorporates the use of electrolyte sheets which are coated on opposite sides with layers of anode and cathode material. Ribbed distributors may also be provided on the opposite sides of the coated electrolyte sheet to form flow channels for the reactant gases. A conventional cross flow pattern is constructed when the flow channels on the anode side of the electrolyte are perpendicular to those on the cathode side. Cross flow patterns, as opposed to co-flow patterns where the flow channels for the fuel gas and oxidant gas are parallel, allow for simpler, conventional manifolding systems to be incorporated into the fuel cell structure. The manifolding system delivers the reactant gases to the fuel cell. The coated electrolyte sheets and distributors of the flat plate design are tightly stacked between current conducting bipolar plates. In an alternate flat plate design, uncoated electrolyte sheets are stacked between porous plates of anode, cathode, and interconnecting material, with gas delivery tubes extending through the structure.
The monolithic solid oxide fuel cell (MSOFC) design is characterized by a honeycomb structure. The MSOFC is constructed by tape casting or calendar rolling the sheet components of the cell, which include thin composites of anode-electrolyte-cathode (A/E/C) material and anode-interconnect-cathode (A/I/C) material. The sheet components are corrugated to form co-flow channels, wherein the fluid gas flows through channels formed by the anode layers, and the oxidant gas flows through parallel channels formed by the cathode layers. The monolithic structure, comprising many single cell layers, is assembled in a green or unfired state and co-sintered to fuse the materials into a rigid, dimensionally stable SOFC core.
These conventional designs have been improved upon in the prior art to achieve high power densities. Power density is increased by incorporating smaller single unit cell heights and shorter cell-to-cell electronic conduction paths. SOFC designs have thus incorporated thin components which are fused together to form a continuous, bonded structure. However, the large number of small components, layers, and interconnections, in addition to complex fabrication steps, decreases the reliability of operational fuel cells.
Difficulties associated with constructing prior art designs, especially in monolithic configurations, occur when various layers of materials having differing compositions and thermal expansion characteristics are co-sintered to form the core of the SOFC. Assembling component parts in a green or unfired state and co-sintering the assembly to fuse the components into a continuous, bonded structure restricts which materials may be selected for use and the thicknesses of the selected materials. Furthermore, considerable thermal stresses arise due to thermal gradients across the cell structure. Defects, such as cracking, can occur during firing, which negatively effect the performance of the fuel cell. Where adjacent cells are fused or bonded together, a single cell which is defectively formed cannot be interchanged with a non-defective cell, and the performance of the assembled fuel cell stack is impaired.
One solution to the above cited problem of micro-cracks developing in component layers is presented in U.S. Pat. No. 4,857,420 to Maricle, which discloses a method of making a monolithic SOFC from finished sub-assemblies. The sub-assemblies, which include electrode sub-assemblies and separator plate-flow field sub-assemblies, are individually sintered to operating size and density prior to being assembled into a SOFC. After the sintered sub-assemblies are layered to construct the fuel cell core, the assembled fuel cell is fused together by heating the fuel cell to a sub-sintering temperature and subjecting the fuel cell to a compressive load (creep flattening), forming a monolithic structure.
An alternative solution is presented in U.S. Pat. No. 5,273,837 to Aitken which discloses the use of pre-sintered ceramic sheets for the construction of electrolyte layers and/or gas channeling structures. Use of pre-sintered flat or corrugated ceramic sheets for cell components provides flexibility, strength, and high thermal shock resistance, characteristics which are not readily achievable in co-sintered prior art designs.
The difficulty of fabricating a continuous, bonded structure with four different materials is similarly addressed in U.S. Pat. No. 5,330,859 to McPheeters, et al., which provides for a SOFC having anode, cathode, and interconnect material comprised of substantially one material.
A low cost, easily fabricated, reliable SOFC is needed for applications where high power density is not critical, and the interchangeability of defective single cell units is desired.
Therefore, in view of the above, a basic object of the present invention is to provide a reliable, low cost method of fabricating a SOFC for applications where a high power density is not required, and wherein a defect in a single cell unit would not permanently impair the performance of the SOFC stack.
A further object of this invention is to provide a design for a single cell unit of a SOFC that is easy to fabricate and allows for the application of conventional manifolding systems utilizing a cross flow pattern for reactant gases.
Another object of this invention is to provide a single cell unit of a SOFC that is individually sintered prior to being assembled into a SOFC stack.
Yet another object of this invention is to connect pre-sintered single cell units in electrical series to form a SOFC stack, whereby the SOFC is not bonded into a continuous structure.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.